Set PKG_CONFIG_PATH if brew command available
This MR assists the building on macOS arm64 (Apple Silicon) where the location of the icu4c dev files are in a directory that is 'non-standard'. For some context, Homebrew on x86_64 machines has its prefix in /usr/local but on arm64, it's /opt/homebrew. Dev files (headers, libs etc) in /usr/local should be picked up automatically when compiling but not /opt/homebrew.
A similar issue with compiling git via gitaly has a similar fix proposed at gitaly!4316 (closed).
Edited by Ash McKenzie